Chapter 3 - Sound Waves

Question 1

What can a Wave do?

Answer 1

A wave can transfer energy from one place to another without a particle being transferred.

Question 2

What is a Transverse wave?

Answer 2

A wave in which the medium vibrates at right angles to the direction of its propagation (growth).

Question 3

Give an example of a Transverse wave.

Answer 3

• Waves that occur on the surface of water.
• When one particle of water is displaced from its zero position by a paddle the adjacent particles also move at a later time because of the intermolecular attraction.
• This time delay between particles is the characteristic of all wave motion.

Question 4

What is a Longitudinal Wave?

Answer 4

• Its particle vibration occurs parallel to the direction of motion of the wave.
• Sound waves propagate in a longitudinal manner.

Question 5

What is the air pressure at the region where the particles are at their position of equilibrium (least energy)?

Answer 5

The air pressure would be normal (1 atmosphere).

Question 6

What is Compression and Rarefaction?

Answer 6

Where particles are pushed close together, this makes a high-pressure zone (compression) and where particles are further apart than the equilibrium position, this is known as rarefaction, or low-pressure zone.

Question 7

Why do sound waves occur?

Answer 7

All sound waves occur because some object is vibrating back and forth, causing air to be compressed (C) and rarefied (R).

eg. When the pressure wave hits the eardrum this causes it to move in and out at the same rate as the high and low-pressure pulses arrive i.e. the same frequency as the source.

Question 8

Explain the movement of a waves.

Answer 8

1. All that each air particle does is to move to the right first (+ direction) as the compression arrives and then left (- direction) as the rarefaction arrives.
2. The maximum displacement of a particle that is oscillating is called its Amplitude (A). Th time for one complete oscillation is called its Period (symbol T).
3. After 1/4 of the time period the particle is in position 2 above which is its furthest point to the right.
4. The particle passes back through the zero position at 1/2 T and moves to a position of negative amplitude after 3/4 of a time period.

Question 9

How can the speed of sound be determined?

Answer 9

By finding how long the sound wave takes to travel a known distance.

e.g. A gun is fired from a hill 2.0km from the gun and a stopwatch started when the smoke from the gun is seen.
The stopwatch is stopped when the sound is heard.

Question 10

Why is this method not accurate?

Answer 10

This method had a large error due to human judgement and reaction time. (about +/- 0.5s)

Question 11

What is a better for determining the speed of sound?

Answer 11

A better method uses the computer to measure the time between when a “click” is emitted from a speaker and when it is received back through a microphone after bouncing back from a wall.

Question 12

Explain the Wave Model of sound created by Christian Huygens.

Answer 12

• It is where light and sound travel through a medium as waves, like waves on water.
• Light waves slow down when entering a denser medium but sound waves actually speed up.
• A point source of sound energy gives out spherical wavelets in all directions, like circular water waves on the surface of a pond.

Question 13

How does each point on a wave act as a new source for secondary wavelets?

Answer 13

Where the wavelets by addition of the wavelet energy cross each other a new wavefront is produced.

Question 14

How do wavelets add up to give another line?

Answer 14

A line of points produce a plane (straight) wave that does not spread out like a circular wave does but only moves in one direction (called a “beam”) ?

Question 15

How is the reflection from a surface explained?

Answer 15

By the difference in time.

Question 16

Concave minor explanation.

Answer 16

• Again the wave model predicts usual observations found in the lab.
• A point focus is where all the wave energy is concentrated into a tiny area.
• A concave reflector can be sued to focus sound.

Question 17

What is a Refraction?

Answer 17

• When light strikes another medium at an angle it changes direction (bends) due to one end of the wave travelling at a different speed to the other end.
• This bending is known as refraction and is caused by a change in velocity.

Question 18

Explain what happens if a sound wave goes into a slower medium according to the wave model.

Answer 18

-The direction will change so it bends towards the normal to the surface and its wavelength decreases but the frequency always remains the same.

Question 19

If the waves strike the interface at right angles then no bending is observed. Why?

Answer 19

Wave explanation:

• This time all parts of the wave front arrive at the same time.
• As the wave slows down the wavelength decreases but the direction and frequency of the wave remain the same.

Question 20

What is Interference of Waves?

Answer 20

It is where one wave meets another, the energy can either add, if the waves are in phase, or subtract if the 2 waves are out of phase.

Question 21

Define Phase.

Answer 21

It refers to the equivalent angle shown by the sine wave at that time.

Question 22

Define Constructive Interference.

Answer 22

In Phase;

-Two wave crests at the same phase angle arriving at a point will combine to give a resultant wave of twice the amplitude.

Question 23

Define Destructive Interference.

Answer 23

Out of Phase;

• A wave crest and a wave trough arriving at a point will combine to give a resultant wave of zero amplitude.

Question 24

How is a Node form?

Answer 24

Through lines of constructive interference; where two high pressure waves coincide.

Question 25

What is a Diffraction?

Answer 25

• It is the curving of a wave around an object.

- The smaller the wavelength, the less diffraction occurs (smaller radius curvature of wavelets).

Question 26

What happens when two sounds with similar amplitudes, but unequal, frequencies are played at the same time?

Answer 26

• One wave peak will coincide with the other at certain times to form a loud sound.
• The points at which the crests and troughs coincide will interfere producing a regular rise and fall in the level of loudness heard.

Question 27

What is Resonance?

Answer 27

If a pulse of energy is given to any rigid object it will cause it to vibrate at its natural frequency of vibration.

Question 28

What is a standing wave?

Answer 28

It is the interference between a wave and its reflection.

Imagine swinging a rope

Question 29

When does a Node occur in swinging a rope?

Answer 29

• There will be points of interference where one part of the rope is being pulled up by the incident wave and pulled down by the reflected wave at the same time.
• This point must be a node because it has no net displacement.

Question 30

When do Antinodes occur?

Answer 30

When the outgoing wave pulls the rope up and immediately afterwards the reflected wave pulls it downwards so a maximum amplitude of vibration is produced.

Question 31

Is Resonance able to occur if a tuning fork is strike and held over the end of a closed pipe?

Answer 31

• Yes it is, provided the length of the tube corresponds to the natural frequency of the tuning fork.
• When the prong of a tuning fork moves downwards it produces a high-pressure zone (compression) which, when it moves upwards, it produces a low pressure zone (rarefaction).